Vital signs of a person, for example the heart rate (HR), the respiration rate (RR) or the arterial blood oxygen saturation, serve as indicators of the current state of a person and as powerful predictors of serious medical events. For this reason, vital signs are extensively monitored in inpatient and outpatient care settings, at home or in further health, leisure and fitness settings.
One way of measuring vital signs is plethysmography. Plethysmography generally refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave traveling through the body of a subject with every heartbeat.
Photoplethysmography (PPG) is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs light more than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform can comprise information attributable to further physiological phenomena such as the respiration. By evaluating the transmittance and/or reflectivity at different wavelengths (typically red and infrared), the blood oxygen saturation can be determined.
Conventional pulse oximeters (also called contact PPG device herein) for measuring the heart rate and the (arterial) blood oxygen saturation (also called SpO2) of a subject are attached to the skin of the subject, for instance to a fingertip, earlobe or forehead. Therefore, they are referred to as ‘contact’ PPG devices. A typical pulse oximeter comprises a red LED and an infrared LED as light sources and one photodiode for detecting light that has been transmitted through patient tissue. Commercially available pulse oximeters quickly switch between measurements at a red and an infrared wavelength and thereby measure the transmittance of the same area or volume of tissue at two different wavelengths. This is referred to as time-division-multiplexing. The transmittance over time at each wavelength gives the PPG waveforms for red and infrared wavelengths. Although contact PPG is regarded as a basically non-invasive technique, contact PPG measurement is often experienced as being unpleasant and obtrusive, since the pulse oximeter is directly attached to the subject and any cables limit the freedom to move and might hinder a workflow.
Fast and reliable detection and analysis of a pulse signal and oxygen saturation level (SpO2) is one of the most important activities in many healthcare applications, which becomes crucial if a patient is in a critical condition, and therefore, the measurement is vulnerable to any sort of artifacts. Further, also in healthy individuals the signals are small and therefore easily distorted by motion of the subject.
Modern photoplethysmography sensors typically fail when the subject moves and the vital signs monitoring is often interrupted. For instance, contact finger pulse oximeters (based on transmissive PPG) are vulnerable to motion of a hand, and fails in case of centralization of a patient due to lower blood volumes on body peripherals. Contact forehead pulse oximeter sensors (using a reflective PPG measurement mode) are supposed to be more robust to a centralization effect. However, the accuracy, robustness and responsiveness of a forehead sensor depends heavily on correct positioning of a sensor on a forehead and proper pressure applied to a skin (too tight application of a sensor might reduce a local blood pulsatility, too loose application might lead to non-reliable measurements due to motion artifacts and/or venous pulsatility).
Recently, non-contact, remote PPG (rPPG) devices (also called camera rPPG device herein) for unobtrusive measurements have been introduced. Remote PPG utilizes light sources or, in general radiation sources, disposed remotely from the subject of interest. Similarly, also a detector, e.g., a camera or a photo detector, can be disposed remotely from the subject of interest. Therefore, remote photoplethysmographic systems and devices are considered unobtrusive and well suited for medical as well as non-medical everyday applications. However, remote PPG devices typically achieve a lower signal-to-noise ratio.
Verkruysse et al., “Remote plethysmographic imaging using ambient light”, Optics Express, 16(26), 22 Dec. 2008, pp. 21434-21445 demonstrates that photoplethysmographic signals can be measured remotely using ambient light and a conventional consumer level video camera, using red, green and blue color channels.
Wieringa, et al., “Contactless Multiple Wavelength Photoplethysmographic Imaging: A First Step Toward “SpO2 Camera” Technology,” Ann Biomed. Eng. 33, 1034-1041 (2005), discloses a remote PPG system for contactless imaging of arterial oxygen saturation in tissue based upon the measurement of plethysmographic signals at different wavelengths. The system comprises a monochrome CMOS-camera and a light source with LEDs of three different wavelengths. The camera sequentially acquires three movies of the subject at the three different wavelengths. The pulse rate can be determined from a movie at a single wavelength, whereas at least two movies at different wavelengths are required for determining the oxygen saturation. The measurements are performed in a darkroom, using only one wavelength at a time.
Using PPG technology, vital signs can be measured, which are revealed by minute light absorption changes in the skin caused by the pulsating blood volume, i.e. by periodic color changes of the human skin induced by the blood volume pulse. As this signal is very small and hidden in much larger variations due to illumination changes and motion, there is a general interest in improving the fundamentally low signal-to-noise ratio (SNR). There still are demanding situations, with severe motion, challenging environmental illumination conditions, or high required accuracy of the application, where an improved robustness and accuracy of the vital sign measurement devices and methods is required, particularly for the more critical healthcare applications.
In summary, Video Health Monitoring (heart rate, respiration rate, SpO2, actigraphy, delirium etc.) is a promising emerging field. Its inherent unobtrusiveness has distinct advantages for patients with fragile skin, or in need of long-term vital signs monitoring, such as NICU patients, patients with extensive burns, or COPD patients who have to be monitored at home during sleep. In other settings such as in a general ward or emergency room, the comfort of contactless monitoring is still an attractive feature. However, while a promising new field, many challenges have to be overcome, in particular designing the system to be robust to movements of the patient is currently one of the main challenges. Further, it is desired to have reliable measurements in case of low pulsatility (e.g. for neonates, at low temperatures). In general, the known methods become unreliable with low SNR, which shall be improved.
A major problem with known methods is their sensitivity for subject motion and low SNR. The reason for this is that the vital signs manifest themselves as small color variations of the skin that are detected by the camera or contact sensor. Subject motion, however, leads easily to much larger variations in the detected signals and recovering the vital sign from these distorted signals is the main issue in the known methods.
G. de Haan and A. van Leest, “Improved motion robustness of remote-PPG by using the blood volume pulse signature”, Physiol. Meas. 35 1913, 2014 describes that the different absorption spectra of arterial blood and bloodless skin cause the variations to occur along a very specific vector in a normalized RGB-space. The exact vector can be determined for a given light-spectrum and transfer-characteristics of the optical filters in the camera. It is shown that this ‘signature’ can be used to design an rPPG algorithm with a much better motion robustness than the recent methods based on blind source separation, and even better than the chrominance-based methods published earlier.
W. Wang, S. Stuijk and G. de Haan, “Exploiting Spatial Redundancy of Image Sensor for Motion Robust rPPG,” in IEEE Transactions on Biomedical Engineering, vol. 62, no. 2, pp. 415-425, February 2015 discloses a framework to improve the motion robustness of rPPG based on the observation that a camera can simultaneously sample multiple skin regions in parallel, and each of them can be treated as an independent sensor for pulse measurement. The spatial redundancy of an image sensor can thus be exploited to distinguish the pulse signal from motion-induced noise. To this end, the pixel-based rPPG sensors are constructed to estimate a robust pulse signal using motion-compensated pixel-to-pixel pulse extraction, spatial pruning, and temporal filtering.
Luis F. Corral Martinez; Gonzalo Paez; Marija Strojnik, “Optimal wavelength selection for noncontact reflection photoplethysmography”, in Proceedings of SPIE—The International Society for Optical Engineering 8011:304. August 2011 describes backscattered signals from human forehead for wavelengths from 380 to 980 nm. The results reveal bands with strong pulsatile signals that carry useful information. Those bands are described as the most suitable wavelengths in the visible and NIR regions from which heart and respiratory rate parameters can be derived using long distance non-contact reflection photoplethysmography analysis. The latter results show the feasibility of a novel technique for remote detection of vital signs in humans. This technique, which may include morphological analysis or maps of tissue oxygenation, is a further step to real non-invasive remote monitoring of patients.